Existing stable isotope tracer methodology was used to monitor the flux through the de novo pyrimidine pathway in vivo. Recovery of pathway activity after acivicin treatment in acivicin-sensitive and acivicin-resistant tumor lines was determined. In conjunction with these studies a new tumor model using invaded spleens was evaluated. Stable isotope tracer methodology was developed for studying the de novo pyrimidine and purine pathways. Methodology was developed using GC/MS techniques in combination with HPLC separations for quantifying isotopic abundances in both purine and pyrimidine bases and nucleosides. In addition, a fully automated GC/MS technique was developed to simultaneously quantify the amount and isotopic 15N enrichment in amino acids in sample extracts of various tissues. A computer model using linear algebra techniques was developed which allow us to interpret data obtained from multiple stable label experiments. The predictive nature of this technique will allow us to study the pharmacologic effects of compartmentation as it relates to antitumor activity of agents which act to inhibit pyrimidine and purine biosynthesis.